1. Field of the Invention
The present invention relates to a micro-optical element used for optical interconnection or the like. In particular, it relates to an optical element using a photonic crystal.
2. Prior Art
Photonic crystals are periodic structures composed of a periodic array of two or more materials of difference refractive indices. The period is corresponding to about half of light wavelength. Photonic crystals are drawing attention because they can highly control the behavior of light. Photonic crystals allow light to be efficiently confined in a spatial domain that is no greater than the wavelength or to be refracted at a steep angle with a low loss. Therefore, if a photonic crystal is used in an optical element, the size of the optical element can be significantly smaller than that of conventional one.
In order to put an optical element using such a photonic crystal into practical use, the optical element has to be optically coupled to an external optical system, such as an optical fiber. In Japanese Patent Application Laid-Open No. 2001-272555 (referred to as literature 1, hereinafter), there is disclosed an art of optical coupling by introducing light into a two-dimensional photonic crystal slab through a surface of the slab perpendicularly from outside thereof.
In addition, in Japanese Patent Application Laid-Open No. 11-72607 (referred to as literature 2, hereinafter) which is laid open in 1999 and Japanese Patent Application Laid-Open No. 2000-171763, there are disclosed optical transmission arts that significantly increase the transmittance of light passing through a series of openings, having a diameter smaller than the wavelength of the light, formed in a metal film.
According to this art, by forming the openings in the metal film in a periodic array, or by giving a periodic configuration associated with the openings to the metal film surface, the intensity of light applied to the metal film and passing through one or more openings in the metal film that have a diameter smaller than the wavelength is significantly increased compared with the metal film having neither such a periodic array of openings nor such a surface configuration. According to experimental verification, it is possible that the light intensity increases by a factor of 1000. It can be considered that such an increase of light intensity occurs when the light incident on the metal film interacts in a resonant manner with the surface plasmon mode excited in the metal film.
According to the method of optical coupling by introducing light into a two-dimensional photonic crystal slab through a surface of the slab perpendicularly from outside thereof, such as that described in the literature 1, the size of a defect formed in the two-dimensional photonic crystal slab to achieve optical coupling is quite smaller than the wavelength of the light used. For example, in case where the wavelength of the light is 1.5 μm, the size of the defect is about 0.5 μm. Therefore, there is a problem that the coupling loss at the defect is extremely high because a normal lens cannot condense light to a size equal to or smaller than the wavelength of the light.
On the other hand, according to the arts disclosed in the literature 2 and the like, light can quite efficiently pass through openings having a diameter smaller than the wavelength of the light. However, the light having passed through the openings spreads out naturally. In those literatures, there is not described any device that allows efficient use (optical coupling) of the light having passed through the openings with a low loss.